Polycarbonate is a thermoplastic resin which possesses a unique set of physical, mechanical, and thermal properties and, hence, may be used in a variety of application areas. Polycarbonates are characterized by high ductility, optical clarity, high heat, and good dimensional stability. Bisphenol A polycarbonate (BPA) is the predominant commercially available resin in this class, which is derived from 2,2-bis (4-hydroxyphenyl)propane, and ordinarily has a glass transition temperature of about 150° C. However, polycarbonates are susceptible to the mechanical forces, especially in outdoor applications, and, thus, may develop scratches and cracks over the time leading to poor surface aesthetics and in some cases, failure of structural integrity.
Healing materials represent a class of the smart materials which have the ability to repair damages. In order to introduce this unique property, polymer substrates may be coated with a healing layer. However, healing layers offer healing properties only to the surface and cannot repair the damages developed in the bulk of the material. Moreover, healing layers often contain solvents and surfactants, which result in a high initial haze of the polycarbonate, making it ill-suited for many applications. Polymer resins may also be mixed with healing agents such as those encapsulated in capsules and catalysts. However, the presence of such foreign materials in the polycarbonate matrix affects other key properties such as transparency. Moreover, such healing systems are driven by irreversible reactions and, therefore, are not effective for repairing multiple events of the damages.
Several approaches have been utilized in an attempt to produce healing polycarbonate materials. For example, U.S. Pat. No. 8,846,801 discloses the use of polyurethane incorporated with carbon nanotubes to introduce intrinsic healing property into polycarbonates while maintaining exceptional mechanical properties and shape-memory. However, the presence of carbon nanotubes may affect other key properties such transparency of the polycarbonate material and limit the applications of the resulting composition. U.S. Pat. No. 7,998,529 discloses haze free and healing coatings on polycarbonate substrates, wherein two isocyanate groups per mole of polycarbonate have been introduced into the polymer backbone and the resulting substrate has been coated with a polyisocyanate healing agents. US Patent Publication No. 2014/0037964 discloses the coating of a healing material on polycarbonate substrates, wherein the healing layer is composed of the mixture of a polycarbonate polyol, a polyisocyanate, a solvent and a surfactant. However, such systems are not capable of multiple healings of damages since the healing mechanism is driven by irreversible cross-linking mechanisms.
It is desirable to develop polycarbonate resins which are capable of healing without any catalysts and can accomplish multiple healings of damages over time.